The present invention relates to fuel cell power generating systems, and to methods of providing electrical power to a load, or to loads at different voltages from a fuel cell power system.
Fuel cells are well known in the art. A fuel cell is an electrochemical device which reacts a fuel and an oxidant to produce electricity and water. A typical fuel supplied to a fuel cell is hydrogen, and a typical oxidant supplied to a fuel cell is oxygen (or ambient air). Other fuels or oxidants can be employed depending upon the operational conditions.
The basic process in a fuel cell is highly efficient, and for those fuel cells fueled directly by hydrogen, pollution free. Further, since fuel cells can be assembled into stacks of various sizes, power systems have been developed to produce a wide range of electrical power outputs and thus can be employed in numerous industrial applications. The teachings of prior art patents, U.S. Pat. Nos. 5,242,764; 6,030,718; 6,096,449, are incorporated by reference herein.
A fuel cell produces an electromotive force by reacting fuel and oxygen at respective electrode interfaces which share a common electrolyte. For example, in PEM fuel cells, the construction of same includes a proton exchange membrane which acts not only as an electrolyte, but also as a barrier to prevent the hydrogen and oxygen from mixing. One commercially available proton exchange membrane is manufactured from a perfluorcarbon material which is marketed under the trademark Nafion, and which is sold by the E.I. DuPont de Nemours Company. Proton exchange membranes may also be purchased from other commercial sources. As should be understood, the proton exchange membrane is positioned between, and in contact with, the two electrodes which form the anode and cathode of the fuel cell.
In the case of a proton exchange membrane (PEM) type fuel cell, hydrogen gas is introduced at a first electrode (anode) where it reacts electrochemically in the presence of a catalyst to produce electrons and protons. The electrons are circulated from the first electrode to a second electrode (cathode) through an electrical circuit which couples these respective electrodes. Further, the protons pass through a membrane of solid, polymerized electrolyte (a proton exchange membrane or PEM) to the second electrode (cathode). Simultaneously, an oxidant, such as oxygen gas, (or air), is introduced to the second electrode where the oxidant reacts electrochemically in the presence of the catalyst and is combined with the electrons from the electrical circuit and the protons (having come across the proton exchange membrane) thus forming water. This reaction further completes the electrical circuit.
The following half cell reactions take place:
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(xc2xd)O2+2H++2exe2x88x92xe2x86x92H2Oxe2x80x83xe2x80x83(2)
As noted above the fuel-side electrode is designated as the anode, and the oxygen-side electrode is identified as the cathode. The external electric circuit conveys the generated electrical current and can thus extract electrical power from the cell. The overall PEM fuel cell reaction produces electrical energy which is the sum of the separate half cell reactions occurring in the fuel cell less its internal losses.
Experience has shown that a single PEM fuel cell produces a useful voltage of only about 0.45 to about 0.7 volts D.C. under a load. In view of this, practical PEM fuel cell power plants have been assembled from multiple cells stacked together such that they are electrically connected in series. Prior art fuel cells are typically configured as stacks, and have electrodes in the form of conductive plates. The conductive plates come into contact with one another so the voltages of the fuel cells electrically add in series. As would be expected, the more fuel cells that are added to the stack, the greater the output voltage.
A typical fuel cell power plant includes three major components: a fuel processor, a fuel cell stack, and a power conditioner. The power conditioner includes a number of components such as, for example, an inverter for converting DC into a 60 Hz AC wave or a DC to DC converter.
A shortcoming with the prior art methods and devices utilized heretofore relates to features which are inherent in their individual designs. For example, fuel cells have been constructed, heretofore, into stack arrangements, the stacks having a predetermined output based upon the number of fuel cells placed together into the stack. In this configuration, there has been no convenient method, apart from controlling the fuel and oxidant supplies to the respective fuel cells, whereby the output of the individual fuel cells within the stack could be accurately and conveniently controlled.
Yet further, fuel cells of the design noted above are relatively slow to respond to increased load demands. For example, when a fuel cell is used in a power distribution system, loads may vary over time. At some times, there may be increased demands, so called xe2x80x9cspikesxe2x80x9d in the load. Because a certain amount of time is usually required to start up a fuel cell stack, additional fuel cell stacks or fuel cell subsystems cannot be instantaneously brought on-line to produce sufficient power to handle these substantially instantaneous spikes in the load. At the same time, a spike in the load that results in an on-line fuel cells capacity being exceeded can potentially damage components of the fuel cell. Thus, fuel cell overcapacity has been provided in prior art systems in order to handle short temporary spikes in the load. This type of design is inefficient and wasteful for obvious reasons.
Fuel cells have, from time to time, been used in conjunction with charge storage devices, such as batteries, which can provide a more instantaneous power supply for given application needs. In most instances, the direct current (DC) power which a fuel cell power system produces, must be converted to alternating current (AC) for many applications. In this regard, an inverter is normally used to convert the fuel cells DC power to AC. In some previous applications, the fuel cell and charge storage device have been coupled to an inverter which functions at the optimal voltage of either the fuel cell or the charge storage devices. In this arrangement, the voltage of the fuel cell was raised or lowered as appropriate, to provide optimum functioning of the system. Still further, experience has shown that altering the voltage resulted in decreased efficiency through heat loss incumbent in the conversion process.
Different customers or users of a fuel cell power plant may require a wide variety of power at different voltages or at different power levels. This could be handled with conventional DC-DC converters, transformers or other power conditioning circuitry; however, these solutions produce losses and inefficiencies inherent in the design of same.
The present invention addresses many of the shortcomings attendant with the prior art practices. For example, some previous designs which provide both a fuel cell and a charge storage device in the arrangement discussed above, have been unduly complex and have experienced decreased efficiency by way of heat losses caused by the conversion of the voltages generated by the fuel cell to make the fuel cell voltage match, as closely as possible, the battery voltage capacity of the charge storage device.
Attention is directed to commonly owned U.S. patent application Ser. No. 09/577,407, which was filed on May 17, 2000 and which is incorporated herein by reference. This application discloses details of one type of ion exchange membrane fuel cell power system having fuel cell subsystems and a controller that could be used in the preferred embodiment of the invention described below.